Base-Promoted Ammonia Borane Hydrogen-Release
The strong non-nucleophilic base bis(dimethylamino)naphthalene (Proton Sponge, PS) has been found to promote the rate and extent of H2-release from ammonia borane (AB) either in the solid state or in ionic-liquid and tetraglyme solutions. For example, AB reactions in 1-butyl-3-methylimidazolium chloride (bmimCl) containing 5.3 mol % PS released 2 equiv of H2 in 171 min at 85 °C and only 9 min at 110 °C, whereas comparable reactions without PS required 316 min at 85 °C and 20 min at 110 °C. Ionic-liquid solvents proved more favorable than tetraglyme since they reduced the formation of undesirable products such as borazine. Solid-state and solution 11B NMR studies of PS-promoted reactions in progress support a reaction pathway involving initial AB deprotonation to form the H3BNH2 - anion. This anion can then initiate AB dehydropolymerization to form branched-chain polyaminoborane polymers. Subsequent chain-branching and dehydrogenation reactions lead ultimately to a cross-linked polyborazylene-type product. AB dehydrogenation by lithium and potassium triethylborohydride was found to produce the stabilized Et3BNH2BH3 - anion, with the crystallographically determined structure of the [Et3BNH2BH3]-K+ · 18-crown-6 complex showing that, following AB nitrogen-deprotonation by the triethylborohydride, the Lewis-acidic triethylborane group coordinated at the nitrogen. Model studies of the reactions of [Et3BNH2BH3]-Li+ with AB show evidence of chain-growth, providing additional support for a PS-promoted AB anionic dehydropolymerization H2- release process.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 979503
- Journal Information:
- Journal of the American Chemical Society, 131(39):14101-14110, Vol. 131, Issue 39; ISSN 0002-7863
- Country of Publication:
- United States
- Language:
- English
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